This invention relates to photometric analysis apparatus in general and more particularly to an improved heat exchange apparatus for use in such apparatus. In a copending application of John G. Atwood et al for U.S. Letters Patent, Ser. No. 594,951, filed July 10, 1975 as a continuation of now abandoned application Ser. No. 499,602, filed Aug. 22, 1974, and assigned to the same assignee as the present invention, there is disclosed a completely automated analysis apparatus for the photometric determination of enzymes in human serum. The determination involves the dilution of the serum and the admixture of two reagents. The reaction mixture is then transferred to the sample cell of a photometer for analysis; specifically, the rate of change of absorbance of the reaction mixture (which is a function of the concentration of the particular enzyme sought) is photometrically determined. In such apparatus, and other similar apparatus, accuracy of results requires that the sample in the photometer cell be maintained at a substantially constant predetermined temperature during the photometric analysis period. It is important first of all that the sample be at a particular temperature within certain tolerances. It is more important, however, that the sample temperature not change during the absorbance measurement period. Thus, to maintain the required accuracy in such apparatus, means are needed to bring the sample up to at least an approximation of a predetermined temperature and to ensure that that sample stays at that temperature throughout the measurement period.